1. Field of the Invention
The present invention relates to a cell search method and apparatus for a mobile station in a mobile communication system.
2. Description of the Related Art
In a mobile communication system based on the CDMA (Code Division Multiple Access) method, if a mobile station communicates with a base station or measures power received from a base station, it must detect frame boundaries and a scramble code in a down signal from the base station. This is called “cell search”.
The cell search method essentially comprises descrambling the signal at all possible scramble codes with all possible timings and selecting a timing and a scramble code with which a correlation coefficient obtained as a result of despreading with spreading codes used is largest, thereby detecting frame boundaries and the scramble code for the base station. This method, however, requires a large amount of time for the cell search. To increase the speed of the cell search, a method is used in which the base station transmits each slot through a PSCH (Primary Synchronization CHannel) and a SSCH (Secondary Synchronization CHannel) (refer to 3GPP Technical Specification 25.211).
FIG. 1 shows a configuration of a down channel relating to the cell search. This down channel relates to the cell search in the W-CDMA method (refer to 3GPP Technical Specification 25.211), which is representative of the CDMA method. On a primary synchronization channel, a spreading code PSC (Primary Synchronization Code) is used, which is common to all cells and slots, and the signals is transmitted in accordance with slot cycles. On a secondary synchronization channel, different spreading codes SSC0 to SSCn−1 (Secondary Synchronization Codes) are used for the respective slots, one frame constitutes a spreading code sequence, and is repeatedly and cyclically transmitted. Different spreading code sequences are used for respective cells. The primary and secondary synchronization channels are not subjected to scramble codes. On a common pilot channel (CPICH), the signals is transmitted by using a spreading code and a symbol pattern which are common to all the cells and subjecting the respective cells to different scramble codes.
FIG. 2 shows a first step operation performed in a conventional cell search method. On the primary synchronization channel, the spreading code PSC, which is common to all the cell and slots, is used. A mobile station inputs a received signal to a matched filter 201 corresponding to this spreading code PSC, and causes an multi-slot averaging section 202 to execute averaging over a plurality of slots in order to reduce the adverse effects of noise and interference. A peak detector 203 selects a timing with which an average correlation coefficient is largest, to detect slot boundaries. This operation is called a “first step”.
On the secondary synchronization channel, the different spreading codes are used for the respective slots, and one frame constitutes a spreading code sequence. The spreading code sequence is repeated in accordance with frame cycles, and different spreading code sequences are used for the respective cells. These spreading code sequences are correlated with respective groups of scramble codes so as to allow the scramble codes to be subsequently detected easily. Since the slot boundaries have been detected at the first step, the mobile station can calculate a transmission timing on the secondary synchronization channel.
The mobile station then despreads a received signal using the calculated timing and the spreading codes SSC, averages correlation output coefficients corresponding to all possible frame boundaries and SSC sequences, and selects a timing and an SSC spreading code sequence with which the average correlation coefficient is largest. The mobile station thus detects the frame boundaries and a scramble code group. This operation is called a “second step”.
FIG. 3 shows a second step operation performed in the conventional cell search method. A timing with which transmissions is executed on the secondary synchronization channel are calculated on the basis of the slot boundaries detected at the first step. A correlator 301 corresponding to the secondary synchronization codes detects a correlation using the calculated timing. This operation is performed over a plurality of slots, and the results are averaged by the multi-slot averaging section 302 so as to correspond to possible timings and SSC spreading code sequences. A peak detector 303 detects frame boundaries and a scramble code group by selecting a timing and an SSC spreading code sequence with which the average correlation coefficient is largest.
The mobile station, which has detected the frame boundaries and the scramble code group during the second step, finally receives the signal, which has been subjected to a scramble code, through the common pilot channel and determines which of the scramble codes of the scramble code group equals that of the signal. Since the frame boundaries have already been detected, the phase of the scramble code can be calculated. Since the spreading code for the common pilot channel is common to all the cells, essentially all the scramble codes within the group are used to descramble the signal, and the spreading code for the common primary channel is used to despread the signal. Then, these operations are performed over a plurality of symbols with the results averaged, and a scramble code is selected with which the average correlation coefficient is largest. This operation is called a “third step”.
FIG. 4 shows a third step operation performed in the conventional cell search method. A correlator 401 calculates the phase of the scramble code on the basis of the frame boundaries detected at the second step, descrambles the signal using all the codes of the scramble code group detected at the second step, and despreads the signal using the spreading code for the common pilot signal. A peak detector 403 selects a scramble code with which the average correlation coefficient is largest, to detect a down scramble code used at the base station.
If the first to third steps are defined as one search, the mobile station determines whether or not the detected frame boundaries and scramble code are correct, after one search has been completed. If it has been determined that they are incorrect, memories for the respective steps are initialized, and the search is restarted. This operation is repeated until the correct frame boundaries and scramble code are detected.
Although a propagation path for mobile communication is subject to interference or noise, the signal noise interference power ratio (S/N) of the received signal of the mobile station on the primary synchronization channel, the secondary synchronization channel, or the common pilot channel is generally very small. On the other hand, the cell search is an operation required if the mobile station is to make various measurements for the signal received from the base station, if it is to communicate, or if it is to carry out handover. The search must be executed promptly and accurately in order to reduce the power consumption associated with the mobile communication and ensure a smooth communication. To increase the speed and accuracy of the cell search, the ratio of power transmitted through the down channel for the cell search to the total power transmitted from the base station may be measured. In this case, however, the system capacity disadvantageously decreases.